AA EXP(=OW Emerald) are entitled to complimentary 500 miles/sticker upgrades at no charge if space is available. Platinum & Gold (Sapphire & Ruby) are next in line but pay $30 per 500 mile sticker for upgrades (earning 4 free for every 10,000 miles flown). According to policy, only after all upgrade requests have been accommodated, will AA sell upgrades.

Much speculation on the last point that AA sells them before upgrading elites. Your story hints that maybe they prioritize the offer to OW partner elites but that is also speculation on my part.